SOAP-BUBBLES. 581 



If we could obtain a body the weight of which was precisely the 

 same as the weight of the water it displaced, it would have no ten- 

 dency to sink or swim, but would remain at rest in any part of the 

 water into which we might choose to place it. Hence this body would 

 be practically free from the attractive influence of the earth, and we 

 should have succeeded in neutralizing the force of gravity, since a 

 body having no tendency to rise or fall might be considered as re- 

 moved to such a distance from the earth as to have no weight. Of 

 course, this conclusion is independent of the fact whether the body 

 introduced into the water is a liquid or a solid, and we may substitute 

 for the water any other liquid ; but, if we employ two liquids, they 

 must satisfy the following conditions : In the first place, they must 

 not mix together, as wine and water do, but must remain separate, 

 like water and oil. In the second place, the weight of any volume 

 oi' one must be exactly equal to that of the same volume of the other; 

 and, in the third place, the two liquids must have, when in contact. 

 no chemical effect upon each other. Could two such liquids be found, 

 a small quantity of the one introduced into a mass of the second 

 would be a state eminently favorable for determining the shape which 

 it would assume under the influence of its surface-tension alone. It 

 would, as I have pointed out, be free from the attraction of the earth, 

 and it would also be free from the force of adhesion to the sides of a 

 solid vessel. It would, however, be extremely difficult to find two 

 liquids which would satisfy these conditions ; but, although we cannot 

 find them to our hand, we are able to manufacture them. Water is a 

 liquid which is heavier than oil, and alcohol is on the other hand 

 lighter than oil ; and, if we mingle water and alcohol, we may make a 

 mixture, the weight of any given volume of which is precisely equal 

 to that of the same volume of oil, and by introducing a few drops of 

 oil into the mass of alcohol and water of the right density we ought 

 to succeed in observing the form which a liquid assumes under the 

 influence of its surface-tension alone. You now see upon the screen 

 the image of a mass of oil in a mixture of alcohol and water of 

 the kind I have just described ; and you see that our question is 

 at once answered the oil assumes* a spherical form. From this we 

 learn that a liquid, if left to the action of its surface-forces alone, 

 will become a sphere. But inasmuch as the effect of the attractive 

 influence of the earth, or the weight of the liquid, increases with 

 the quantity we use, while on the other hand the surface-tension, or 

 its own moulding molecular force, remains precisely the same, we 

 should, if we use a large quantity of liquid, expect the weight to be 

 the particular force which determined its shape ; and if Ave employ a 

 small mass of liquid, then the surface-tension, growing proportionately 

 greater, would become the more important. Thus it follows that, 

 although we have to use the most accurate adjustment in order to 

 obtain a sphere of oil an inch in diameter, every rain-drop, every dew- 



